1. Field of the Invention
The present invention relates to a device for preconditioning farinaceous materials such as soy-containing pet foods prior to treatment in an extrusion cooker. More particularly, the device is concerned with a conditioning vessel having two, juxtaposed, frustocylindrical chambers and respective axially mounted, rotatably driven, mixing shafts having mixing elements extending therefrom and configured to present sequential, differentiated, conditioning zones.
2. Description of the Prior Art
Preconditioners are widely used in combination with extruders for preparing and blending food materials before further processing and cooking in an extruder. For example, products having a relatively high percentage of flour-like material are often blended with water and treated with steam in a preconditioner prior to extrusion. Use of preconditioners is particularly advantageous in preparing products composed of farinaceous material such as pet food containing a relatively large percentage of soy flour.
Some prior art preconditioning devices include an elongated vessel having a pair of identical side-by-side, frustocylindrical, intercommunicated mixing chambers. Each chamber is provided with an axially mounted shaft having mixing elements extending radially outwardly therefrom. The mixing elements are configured for advancing the material from an inlet end of the vessel toward an outlet end and for sweeping the material around the frustocylindrical walls to cause exchange of material between chambers.
A series of liquid inlets are often provided along at least a portion of the length of preconditioning vessels for adding water or other liquid such as fat to the food material during advancement through the mixing chambers. Obviously, it is highly important that any liquid introduced into a preconditioning vessel become thoroughly and uniformly blended with farinaceous material to avoid formation of clumps. Typically, clumps represent a nonhomogeneous mixture of the material and liquid wherein the material forming the outer surface of the clump presents the highest percentage of moisture. Proper blending of liquid with farinaceous materials requires both proper mixing or agitation of the liquid and materials, and sufficient residence time within the preconditioning vessel to ensure equilibration.
Increasing the rotational speed of the mixing elements of conventional preconditioners in an attempt to increase agitation within the vessel causes the material to pass through the vessel at a greater speed which correspondingly reduces the residence time of the material within the vessel to unacceptable levels. On the other hand, reducing the rotational speed of the beaters to increase residence time within the vessel can adversely affect the mixing characteristics of the vessel to the point where proper blending of the material with liquid may not be achieved.
Furthermore, increasing the overall length of the vessel is not desirable because of mechanical problems associated with longer mixing shafts. Moreover, the structural nature of conventional preconditioning devices may not provide operational flexibility for preconditioning different materials at varying flow rates.
U.S. Pat. No. 4,752,139, which is hereby incorporated by reference, discloses a preconditioning apparatus which provides operational flexibility and improved preconditioning in many circumstances. It has been found, however, that some mixtures do not receive both adequate mixing and retention time in the '139 apparatus.